Bolts from the blue

By Keay Davidson

“I recently witnessed the most spectacular and unexpected natural event I have seen,” wrote commercial pilot John Hammerstrom of Tavernier, Florida, in 1993. When he was flying over Central America, Hammerstrom had glimpsed strange jets of blue light springing up from a thundercloud. Such jets, and the giant flashes dubbed “sprites” that dance in the upper atmosphere above thunderstorms, have been baffling atmospheric researchers since their first reliable sightings in 1989. And this summer, a host of new projects are under way in the US to try to understand what processes are at work.

While it’s clear that they are intimately associated with thunderstorms, sprites are not lightning as we understand it. They appear as giant nebulous glows that occur up to about 90 kilometres above the Earth. Their shapes are complex and diverse; observers have described them as carrot and turnip-shaped, or “jellyfish”. The light is usually red, sometimes with blue tentacles, and lasts for just a tiny fraction of a second. They usually occur in clusters of two, three or more.

Other strange atmospheric glows are blue jets that burst from the tops of thunderclouds, much lower down in the atmosphere. The jets propagate upwards in cones about 15° wide, and at speeds close to 100 kilometres per second. They are often trumpet-shaped, flaring out at the top as they reach their maximum altitude. But it’s all still a mystery – no one knows where the energy that fuels these giant ghosts comes from, or whether blue jets and sprites have related causes. “This is totally weird science,” says atmospheric researcher Walter Lyons of ASTeR, …

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